A semiconductor wafer polishing apparatus has been known as an example of a work processing apparatus. In the semiconductor wafer polishing apparatus, a surface of a wafer is polished by steps of: bringing the surface of the wafer held by a wafer holding plate (e.g., a carrier) of a polishing head; and relatively moving a polishing plate and the polishing head, with respect to each other, with supplying a polishing liquid from a polishing liquid feeding unit to a polishing cloth adhered on an upper face of the polishing plate.
An example of a conventional polishing liquid feeding unit (i.e., a slurry feeding unit) of the semiconductor wafer polishing apparatus is disclosed in Patent Document 1.
In a process of polishing a semiconductor wafer, a polishing liquid (i.e., slurry) is dried on surfaces of liquid paths of the polishing apparatus through which slurry and pure water flow, a wafer processing section thereof, the wafer to be polished, etc., so the dried solid slurry causes following serious problems: lowering polishing performance; lowering flatness of the surface of the wafer; and making flows of liquids in the apparatus and liquid tanks worse. To avoid the problems, a method of automatically replenishing a slurry, in which the slurry in a slurry tank is automatically suitably replenished so as to prevent the slurry tank from being dried, prevent a liquid surface of the slurry from being lowered and prevent the slurry from being solidified, has been known.
For example, the slurry feeding unit disclosed in Patent Document 1 has a liquid surface sensor or a load cell for detecting a residual quantity of the slurry in the slurry tank so as to automatically replenish the slurry and automatically stop replenishing the slurry. Further, the slurry is replenished in accordance with the replenishing sequence, by using a timer, before the slurry is dried on an inner wall surface of the slurry tank, and the inside of the tank is humidified by using a spray nozzle or a humidification unit at an arbitrary time interval.